1. Field of the Invention
This invention relates to the art of geophysical exploration using artificial seismic energy, and more particularly, to apparati and methods for seismic exploration of onshore regions that are relatively inaccessible to vehicles.
2. Description of the Prior Art
Geophysical prospecting using aritificially induced seismic disturbances has found wide application in the search for petroleum and other valuable resources. It is the general practice to generate energy at a source point near the surface of the earth and to direct seismic energy into the earth from that point. The energy returning from within the earth is sensed and recorded. The seismic trace thus recorded is processed and then interpreted to provide an indication of the structure of the underlying strata.
In the exploration of regions which are relatively inaccessible to vehicles, the weight of supplies and equipment required determines the practicality of a particular exploration system. At the present time, a wide variety of low energy seismic exploration systems are available. In these systems, a coded energy signal is transmitted into the earth and the raw seismic data which is obtained is correlated according to the code of the coded energy signal. The record, or signature, of the source code must be of very good quality in order to obtain a good quality correlated trace. The low energy systems can be generally classified according to the method employed to obtain the source signature as either a master-type or a slave-type source system. In the master-type source systems, the source signature is sensed as the coded energy signal is transmitted. In the slave-type source systems, the source signature is predetermined and a slave source is commanded to transmit that signature.
In the master-type source system disclosed in French Pat. No. 2,247,734 the source emissions are controlled manually within the physical limits of the powered impactor source. Powered impactor sources are difficult to control in a manner which will produce a good source code because they tend to operate at certain natural frequencies and are subject to undesirable residual bounces. Master-type source systems employing rotating eccentric weight sources are disclosed in U.S. Pat. No. 3,185,250 to Glazier, U.S. Pat. No. 3,244,252 to Burns, and U.S. Pat. No. 2,921,477 to Hanggi. In all the prior art master-type source systems, the source signature is monitored by a sensor, such as an accelerometer or geophone, which is located on or near the source. While these master-type source systems of the prior art are relatively light-weight and would therefore be well suited to exploration in regions not accessible to vehicles, they have not been used successfully because the source signatures obtained by the prior art sensors are attenuated and phase shifted and usually contain significant interferences. Cross-correlation of the seismic trace with this poor quality source signature yields poorly resolved processed traces. Consequently, numerous methods have been devised to extract the true source code from the source signature. However, these methods have only been moderately successful.
On the other hand, the processed traces obtained by use of the slave-type source systems have much better resolution. Because the source transmits energy according to a predetermined code, the code is known and need not be recorded by use of a sensor. Furthermore, carefully designed source codes, which yield high resolution seismic data, can be transmitted by precise control of the source. The slave-type source systems, such as the well known VIBROSEIS.RTM. system developed and licensed by Continental Oil Company, Pona City, Oklahoma, have been relatively successful. However, the weight of the equipment required, specifically the heavy master control and vibratory employed, increases markedly as the resolving power of these systems is enhanced. Since the VIBROSEIS.RTM. and similar type sources must be coupled to the ground, their peak force to weight ratio must be less than 1. The best of these sources are normally vehicle mounted and weigh between about 10 and about 20 tons. Due to this great weight, slave-type source systems are impractical for use in regions not accessible to vehicles.
Due to the various inadequacies and limitations of these prior art low energy, multiple emission systems, exploration in regions not accessible to vehicles is currently performed with the more primitive single-shot explosive systems. These systems are preferred even though they require seismic and drilling crews of typically between 100 and 500 men to carry the equipment and supplies. In many regions, these explosive systems are prohibited due to their adverse impact on the environment. Clearly a need exists for a light weight, high resolution seismic exploration system for use in regions not accessible to vehicles.
Accordingly, a primary object of this invention is to provide a lightweight and high resolution seismic exploration system.
Another object of this invention is to provide a seismic source which includes means for sensing the time breaks of the source impulses.
A further object of this invention is to provide a master-type seismic source which includes a sensor for providing an accurate and noise-free source signature.
Yet another object of this invention is to provide a seismic exploration method in which the required weight of the exploration equipment is reduced without sacrificing seismic data quality.
Another object of this invention is to provide a seismic exploration system for use in relatively inaccessible onshore applications in which the source means and the data handling method employed cooperate to reduce the overall weight requirement of the system.
Other objects and advantages of this invention will become apparent to those skilled in the art from the following description.